Another vote for the rotozip. I rarely use it, but every few years when I need to cut a hole in drywall, I use it. Mine came with a clear plastic vacuum attachment that catches 99% of the dust, and that's the main reason why I use it.
And as far as cutting through a plastic electrical box with one - sure, you can do it, but for those not familiar with the rotozip bits: the ones for drywall have no flutes for the first 3/16" on the end of the tip. This is the guide portion of the bit. You set the depth of the bit such that only this smooth portion of the bit tip comes through the drywall (or maybe just a bit further).
So when you have the drywall flush up against the electrical box, the idea is that you plunge through the drywall in the center, and then zip out until you hit the inside edge of the box (so you only have to be +/- 1" on the center of the box marked on the drywall to start with). Then you pull out slightly, and then put the bit back in when it's on the outside edge of the box - then you follow the outside edge of the box all the way around.
The key is to use light pressure, and not to force the bit - you do have to fight the spin of the bit, but once you get used to that, it's not bad. You will quickly find out which direction to go around the box for best results.
And for me, since I don't do this very often, I really get a kick out of using the rotozip since it works so well. They aren't that expensive, so my vote is to get one and practice a bit with it first (the local big box stores give away partial pieces of drywall for free, at least in my area).
